Trading server delay is always present if your trading terminal, e.g. MetaTrader, communicates to the trading server over the Internet. This data exchange channel has an inherent delay, caused both by the physical distance to the server and by the delays in processing of the signal by the numerous switches on the way.
Due to the delay, you can not always expect to place an order at the price that you see on your computer. This price was in effect at least a RTT ago. RTT, or Round Trip Time, is the time needed for your trading terminal to send a request for the current price to the trading server, and get the response back.
What are the delays? For very good network conditions, the delay can be less than 100 ms. In most cases, the delay is measured in several hundred milliseconds, but sometimes it can exceed 1 s.
You can estimate the delay yourself. On the screenshot below, you can see how to do it in your Windows computer. The command for that is: ping <IP-address>
In this example, we are measuring the delay to our WiFi access point (192.168.254.254). This is the best posssible situation and the delay is very small.
If you uses the IP-address of your trading server, you can see your delay.
So when you see the price that is at least RTT is delayed by at least the same time. So the price you target when you click the order button, and the price in effect when your order reaches the trading server, will most probably differ. And on fast moving markets, the difference can be considerable.
This is taken into account in the Forex Simulator. You can configure the delay randomly varying between min and max values. And try how the trading feels for different delay values.